The invention relates to a system for reducing vibrations in the cabin of an aircraft driven by two or more propellers each having n blades (n being an integer equal or larger than 2), the relative phase angle of which may be adjusted, the system comprising at least
synchrophase means for adjusting the relative phase angle such that a predetermined relative phase angle may be maintained; PA1 vibration level detecting means to supply information about the vibration level at one or more positions within the cabin or information related thereto, and to generate a corresponding vibration level signal, and PA1 a control unit supplying an error signal to the synchrophase means in response to the vibration level signal causing the synchrophase means to adjust the predetermined relative phase angle to a new value causing a reduced vibration level. PA1 synchrophase means for adjusting the relative phase angle such that a predetermined relative phase angle may be maintained; PA1 vibration level detecting means to supply information about a vibration level at one or more positions within the cabin or information related thereto, and to generate a corresponding vibration level signal, and PA1 a control unit supplying an error signal to the synchrophase means in response to the vibration level signal causing the synchrophase means to adjust the predetermined relative phase angle to a new relative phase angle causing a reduced vibration level, the new relative phase angle differing m.(2.pi./n) radians (m being an integer) from the predetermined relative phase angle and wherein each propeller comprises at least one recognising means attached to one propeller blade, each recognising means being detectable by a respective sensor connected to the control unit to send a detection signal to the control unit upon passing of the recognising means past the respective sensor.
Examples of such systems are given by U.S. Pat. Nos. 2,847,617; 2,877,855; 2,878,427; 3,007,529; 3,589,832; 4,245,955; 4,653,981; 4,659,283 and 4,703,908; European application EP 0,322,343; Canadian Patents 0,664,628 and 0,472,689 and the UK Patent 2,211,635. The object of these prior art systems for propeller synchrophasing is to maintain an accurate positional or phasic agreement between one blade of a first propeller, which is named the master, and any blade of the other propeller(s), which are named the slaves.
An other example of such a system is known from EP-0,252,647, which discloses an apparatus that may reduce cabin noise levels by continuously sensing during the flight the vibration level in the cabin and calculating therefrom the optimum phase relationship between the (master) propeller and at least one other (slave) propeller. The control unit supplies an error signal to a synchrophaser which adjusts the blades of the slave propeller(s) to a position of minimum vibration level starting from one specific begin phase relationship.
A source of discomfort for passengers and crew of propeller-driven aircraft is the action of the various engines and propellers generating annoying vibrations and dominant noise in the cabin and cockpit. A propeller with a rotational speed N and a number of blades n, generates excitation forces having frequencies of N Hz and nN Hz with harmonics. Vibrations of this nature have regular and irregular patterns. In order to reduce the vibration and internal noise to an appreciable level for crew and passengers, modern propeller driven multi-engined aircraft utilize all kind of energy absorbing, vibration isolating and reactive force devices to reduce the transmission of mechanical vibrations from the propulsion units to the fuselage.
For example, isolators are often mounted in the connection between engine and engine support to isolate the engine/propeller combination from objectionable dynamic foundation displacements, and/or isolate the aircraft structure from objectionable dynamic foundation displacement, and/or isolate the aircraft structure from objectionable dynamic forces from the engine/propeller combination. Such apparatus include elastomeric and metal spring elements for dampening vibration in one or more directions.
The complex factors determining the human response to vibrations and the paucity of consistent quantitative data concerning man's perception of vibration and his reactions to it, are difficult to translate in generally recognizable criteria of comfort or discomfort. However, when the intensity, frequency and duration of the vibration at several locations in the cabin, and also consequences of such vibrations like visible vibration of passenger seats or tables are judged by a variety of passengers in comfort or discomfort, acceptance criteria can be derived. From a survey of many aircraft an acceptable maximum level of the energy of the vibration at several locations in the cabin was established.
During first flights of Fokker 50 series aircraft, which is a twin engined propeller-driven passenger aircraft, it appeared that some of the aircraft would expose future passengers to uncomfortable vibrations. With the help of above mentioned acceptance criteria, the influence of modified vibration engine isolators was investigated. Also investigated was the influence of minimizing the propeller mass unbalance. Results of both investigations indicated that the vibration level could be reduced for the given new-built aircraft. However, after delivery of the aircraft to the operator maintenance of the aircraft would change the configuration, for example by the replacement of a propeller. Therefore the cause of vibrations and noise in the cabin which affect passenger comfort in a negative way, had to be solved more fundamentally.
A further reason to proceed investigations was the observation that despite modified isolators and minimized unbalance, during flight after an engine shut down and relight a period of sufficiently low energy level of vibrations before the shut-down was succeeded by a period with a higher level after the relight, and vice versa. Continued measurements during flight tests whereby the phase relationship between the two propellers of the aircraft was controlled by the propeller synchrophase system, showed a relation between the vibration level and the changes of the position of the blades of the starboard propeller with respect to the blades of the port propeller. From these observations distinctions in detail were made between the vibration and noise level in the cabin before and after the synchrophase system had readjusted upon a disturbance the required phase relationship between both propellers.
By measurements of the relation between the vibration levels in the cabin and the relative position of the blades of the right hand propeller versus the left hand propeller, it was found that when the position of a particular blade of one propeller and the position of a particular blade of the other propeller were synchrophased, the vibration level in the cabin showed a minimum. Also the contrary was measured, namely that a maximum vibration level could occur by synchrophasing two particular other blades. The findings were worked out in an add-on system of the prior art propeller synchrophase system to arrange that the add-on system in conjunction with the synchrophase system selects automatically the pair of blades from the propellers of the aircraft that gives the minimum vibration level in the cabin, and through that an improved comfort for crew and passengers. The add-on system does not effect the above mentioned specific function of the synchrophase system itself.
A propeller blade position control system to reduce the vibrations and noise in the cabin as much as possible is disclosed in U.S. Pat. No. 5,150,855 assigned to the Assignee of the present patent. In the system described in U.S. Pat. No. 5,150,855, during adjustment of the predetermined relative phase angle, i.e. the phase angle between the propeller blades of one propeller and corresponding propeller blades of another propeller (or other propellers), by the synchrophase system the propeller blades of one propeller are accelerated or decelerated in order to obtain a new phase relation between them and the propeller blades of another propeller (or other propellers). The new phase relation differs from the original one by m.(2.pi./n) radians. This system is very effective under most conditions, but it would be desirable to provide means to accommodate localized disturbances affecting the propellers unequally. For example, during adjusting or already before during the measurement procedure to establish an optimum relative phase angle, one of the propellers might be affected by atmospherical disturbances not affecting the other propeller (or propellers) in the same way. Such disturbances might introduce an uncontrolled acceleration or deceleration of one propeller with respect to another propeller (or the other propellers). Without additional measures the vibration level detecting means and the control unit may have difficulty establishing a new relationship after the disturbance occurred. In other words, it is difficult to detect the exact additional relative phase shift caused by the disturbance. After an entire measurement procedure the relative phase shift ought to be substantially 2.pi. radians, however, the disturbance might have caused in the particular circumstances of unequal, localized disturbances, an unknown additional relative phase shift. Therefore, establishing the optimum relative phase angle may be difficult.